Semiconductor storage device such as a NAND flash memory device is facing increasing demand for further microfabrication. Narrower memory cell requires smaller spacing between the neighboring memory cell gate electrodes and thereby increases cross coupling effect of the memory cell gate electrodes to an unignorable level. Cross coupling effect is typically observed through a shift in the threshold voltage of a programmed memory cell transistor by the influence of the subsequent programming of adjacent memory cell transistors. Cross coupling effect expands the range of threshold voltage distribution and thus, may lead to erroneous data read out.
Increased capacitive coupling between adjacent memory cell gate electrodes originating from narrowed memory cell gate electrode gaps increases the magnitude of cross coupling effect. On solution may be forming air gaps between the memory cell gate electrodes to reduce the capacitive coupling between the adjacent memory cell gate electrodes.
Formation of air gaps between the adjacent memory cell gate electrodes also forms air gaps between select gate electrodes and contacts. Such configuration risks the possibility of contact shorting by errors such as etching errors and misalignment. More specifically, when the contact hole is unwantedly connected with the air gap formed in the proximity of the select gate electrode, the contact material may be introduced into the air gap to cause shorting of the adjacent contacts, which in turn leads to yield degradation of the memory being manufactured.